This invention relates to a torque sensor and a method for manufacturing the same, more particularly to a torque sensor which permits measuring a large torque with a good accuracy in an extensive temperature range, and a method for manufacturing such a torque.
In recent years, it has been required to accurately detect a torque of a rotary member. For this requirement, a non-contact system, where a detector is not brought into contact with the rotary member, is suitable.
Heretofore, as the non-contact systems, there have been attempted an indirect system by which a torosion angle in a shaft is detected with the aid of beam or magnet to indirectly measure the torque, and a direct system in which a magnetic element is disposed on the rotary member and the torque is detected by the utilization of a magnetic strain phenomenon caused by the rotation of the magnetic element. However, they cannot be used practically.
As compared with the indirect system, the aforesaid direct system permits easily detecting the torque in stationary, normally rotating and reversely rotating states, and it is thus preferable from the viewpoint of usefulness. However, in the case of the conventional direct systems, it is difficult to detect the accurate torque because of the ununiformity of magnetic properties of the magnetic element.
Now, a torque sensor has lately been suggested by which the torque is directly detected in a non-contact style by the utilization of magnetic strain properties of an amorphous magnetic alloy (Data for Magnetics Study Meeting in Denki Gakkai, MAG-81-71).
Referring to the instant torque sensor, a thin strip of an amorphous magnetic alloy having great magnetic strain properties is wound and fixed on a rotary shaft so that the strain in the shaft caused by the torque may be introduced into the thin strip above, and a variation in the magnetic properties of the thin strip by a magnetic strain phenomenon is detected in the non-contact style from outside, thereby measuring the torque. In order to provide the thin strip of the amorphous magnetic alloy with great magnetic strain properties (inductive magnetic anisotropy), the annular magnetic core of the amorphous magnetic alloy thin strip is first prepared so that its diameter may conform to the diameter of the rotary shaft and is then subjected to heat treatment to remove an internal stress from the thin strip. Afterward, this thin strip is adhesively fixed on the rotary shaft which is distorted, and the torsion therein is then cancelled by a return operation, thereby providing the thin strip with the inductive magnetic anisotropy.
The above-mentioned torque sensor will be described in brief with reference to accompanying drawings.
As shown in FIG. 1, the aforesaid torque sensor has an annular magnetic core 2 comprising an amorphous magnetic alloy thin strip insertedly fixed around a rotary shaft 1. Now, when a torque 3 is applied to the rotary shaft 1, a strain stress will occur therein at an angle of .+-.45.degree. to the lengthwise axis thereof, so that a strain stress .sigma.4 will occur also in the annular magnetic core 2 absolutely fixed on the rotary shaft 1, at an angle of .+-.45.degree. to the lengthwise axis thereof, as shown in FIG. 1. Further, if the annular magnetic core is used to which uniaxial anisotropy Ku 5 is applied, for example, at an angle of .theta.=45.degree. as shown in FIG. 2, the anisotropy Ku 5 will be changed into Ku' 6 by the strain stress .sigma. which has been caused by the application of the above-mentioned torque 3. Therefore, the torque applied to the rotary shaft can be measured by electrically detecting a change amount of the above-mentioned uniaxial anisotropy.
In the aforesaid torque sensor, when the thin strip of the amorphous magnetic alloy which has a great magnetic strain constant and a high saturated megnetization has been used, an output voltage to be detected will become large, which fact advantageously permits its measurement with a high accuracy. Further, if a great torque T.sub.0 is previously applied to the thin strip of the amorphous magnetic alloy, a greater torque T can also advantageously be detected.
However, the torque sensor described above suffers certain disadvantages, in part because the amorphous magnetic alloy used (trade name: Metaglas 2826MB; available from U.S. Allied Co., Ltd.; Fe.sub.40 Ni.sub.38 Mo.sub.4 B.sub.18) is poor in its performance. That is to say, when the thin strip comprising such a poor amorphous magnetic alloy is used as an annular magnetic core and when a heat treatment of the thin strip is carried out to remove an internal stress therefrom, the thin strip of the amorphous alloy itself will become very brittle. As a result, it has problems that some cracks tend to occur at a time of a torque measurement, and when a great torque T.sub.0 is applied to the thin strip of the amorphous magnetic alloy with the aim of detecting a great torque, some crack will occur in the annular magnetic core consisting of the thin strip of the amorphous magnetic alloy.
Moreover, fixing the thin strip of the amorphous magnetic alloy on the rotary shaft is carried out with the aid of an adhesive such as a synthetic resin. Therefore, if it is attempted to detect the torque in the range of an elevated temperature, the adhesion between the alloy thin strip and the rotary shaft will deteriorate along with temperature raise. For this reason, the stress in the rotary shaft will not be accurately transmitted to the thin strip of the amorphous magnetic alloy, and a detection output for the torque will fluctuate, which fact will lead to the deterioration in the detection accuracy of the torque.
Further, in a manufacturing process, it is required to prepare the annular magnetic core the diameter of which is previously caused to conform to the diameter of the rotary shaft, and a torsion must be applied to the rotary shaft. These requirements make the manufacturing process intricate.